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September 15, 2013 05:38
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Freeモナドのスコット・エンコーディングによる実装ですー>ω< kazu-yamamoto氏のhttps://gist.github.com/kazu-yamamoto/4064634#file-gistfile1-hs-L76を参考にさせていただきましたのです(`・ω・´)
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{-# LANGUAGE Rank2Types, DeriveFunctor, LambdaCase #-} | |
module Control.Monad.Free.Scott (($|), Free(..), free', pure', liftF) where | |
infixl 1 $| | |
($|) :: (a -> b) -> a -> b | |
f $| x = f x | |
-- Type | |
newtype Free f a = Free { | |
matchFree :: forall r. (f (Free f a) -> r) -- Free (f (Free f a)) | |
-> (a -> r) -- Pure a | |
-> r } | |
-- Constructors | |
free' :: (f (Free f a)) -> Free f a | |
free' x = Free $ \f p-> f x | |
pure' :: a -> Free f a | |
pure' x = Free $ \f p-> p x | |
-- Instances | |
instance Functor f => Functor (Free f) where | |
fmap f x = matchFree x | |
$| (\y -> free' $ fmap (fmap f) y) | |
$| (\x'-> pure' $ f x') | |
instance Functor f => Monad (Free f) where | |
return x = pure' x | |
x >>= f = matchFree x | |
$| (\y -> free' $ fmap (>>= f) y) | |
$| (\x'-> f x') | |
-- Functions | |
liftF :: Functor f => f r -> Free f r | |
liftF = free' . fmap pure' |
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{-# LANGUAGE Rank2Types, DeriveFunctor, LambdaCase #-} | |
import Control.Monad.Free.Scott | |
import Test.QuickCheck (quickCheck) | |
type CharIO = Free CharActions | |
data CharActions a | |
= PutCh Char a | |
| GetCh (Char -> a) | |
deriving (Functor) | |
putCh :: Char -> CharIO () | |
putCh c = liftF $ PutCh c () | |
getCh :: CharIO Char | |
getCh = liftF $ GetCh id | |
runCharIO :: CharIO a -> IO () | |
runCharIO x = matchFree x | |
$| (\case | |
PutCh c x'-> putChar c >> runCharIO x' | |
GetCh f -> getChar >>= runCharIO . f) | |
$| (\_-> return ()) | |
echo :: CharIO () | |
echo = do | |
getCh >>= putCh | |
echo | |
main = runCharIO echo | |
data Output a | |
= Print Char (Output a) | |
| Read (Output a) | |
| Return a | |
| EOF | |
deriving (Eq, Show) | |
toOutput :: CharIO a -> String -> Output a | |
toOutput x cs = matchFree x | |
$| (\case | |
PutCh c x'-> Print c (toOutput x' cs) | |
GetCh f | |
|null cs -> EOF | |
|otherwise -> Read $ toOutput (f $ head cs) $ tail cs) | |
$| (\x-> Return x) | |
copy :: String -> Output a | |
copy "" = EOF | |
copy (c:cs) = Read $ Print c $ copy cs | |
propEcho :: String -> Bool | |
propEcho s = toOutput echo s == copy s | |
test :: IO () | |
test = quickCheck propEcho |
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